In a piezoelectric ink jet printer, a print head includes a large number of ink chambers, each of which is in fluid communication with an orifice and with an ink reservoir. At least one wall of the ink chamber is coupled to a piezoelectric material. When actuated, the piezoelectric material deforms. This deformation results in a deformation of the wall, which in turn launches a pressure wave that ultimately pushes ink out of the orifice while drawing in additional ink from an ink reservoir.
To provide greater density variations on a printed image, it is often useful to eject ink droplets of different sizes from the ink chambers. One way to do so is to sequentially actuate the piezoelectric material. Each actuation of the piezoelectric material causes a volume of ink to be pumped out the orifice. If the actuations occur at a sufficiently high frequency, such as at resonant frequency or at a frequency that is higher than the resonant frequency of the ink chamber, and at appropriate velocities, successive volumes will be pumped out of the orifice and will combine in flight to form a single drop on the substrate. The size of this one droplet depends on the number of times actuation occurs before the droplet begins its flight from the orifice to the substrate.